1. Field of the Invention
The present invention relates to a method and a device for heat-processing a plurality of semiconductor wafers while preventing them from being oxidized. The present invention also relates to a boat for use with the heat process device.
2. Description of the Related Art
Semiconductor devices such as IC and LSI are manufactured after wafers cut from ingot are subjected to various process such as surface machining, heat oxidizing, impurity diffusing, film depositing and etching. The semiconductor wafers are heat-processed several times int eh course of these various processes.
In the film depositing process, for example, the semiconductor wafers are mounted on boats made of quartz, loaded into process tubes in diffusion furnaces and reacted with process gas under high temperature. A lot of wafers mounted on a boat usually reaches up to 200 pieces.
When the boat is loaded and unloaded into and out of the process tube in the diffusion furnace, an exclusive fork made of quartz is used. Systems for driving the fork in this manner are of the cantilever and soft landing types. In the case of the cantilever system, the fork is inserted into the process tube while keeping the boat not to contact the process tube and heat process is conducted while supporting the boat by the fork. In the case of the soft landing system, the boat is soft landed into the process tube from the fork, the fork is kept outside the furnace while heat process is being conducted, and the fork is again inserted into the furnace to carry the boat out of the furnace after heat process is finished.
Before the boat is loaded into the furnace, the furnace is filled with non-oxidizable gas such as nitrogen gas to prevent the wafers from being oxidized. Whichever of the cantilever and soft landing systems may be employed, air enters into the furnace, when the fork is inserted into and released from the furnace, to thereby cause unnecessary oxide film to be formed on the surface of each of the wafers. This is quite undesirable.
In the case of the conventional heat process device as shown in FIG. 1, therefore, scavenger 3 is located between loading/unloading unit 1 and heat process furnace 2 and boat 6 is inserted into and released from process tube 10 in the furnace through auxiliary air-tight tube (so-called elephant tube) 14 in scavenger 3. Heater 12 is located around process tube 10 to heat wafers in tube 10. Process tube 10 and auxiliary air-tight tube 14 are air-tightly connected to each other and process or nitrogen gas is introduced into tube 10 through inlet 11 while nitrogen gas into tube 14 through inlet 16. Shutter 15 is located at the open end of auxiliary air-tight tube 14 and boat 6 is inserted into and released from auxiliary air-tight tube 14 by means of fork 5 at soft landing section 4 while closing and opening the open end of auxiliary air-tight tube 14 by shutter 15.
In the case of this heat process device, gas in scavenger 3 is previously exhausted through outlet 17 and nitrogen gas is introduced into auxiliary air-tight tube 14 through inlet 16. This efficiently prevents air from entering into process tube 10.
However, auxiliary air-tight tube 14 is made continuous from and communicated with process tube 10 with no partition wall interposed between them. Therefore all of the air cannot be eliminated from auxiliary air-tight tube 14, thereby allowing air to enter into process tube 10. This causes wafers 7 to be oxidized in the furnace, preventing desired film from being formed on the surface of each of wafers 7.
This problem is also caused when boat 6 is unloaded from furnace 2. When wafers 7 on which film has been formed are put outside immediately after the heat process, the film on wafers 7 is oxidized. Boat 6 must be therefore unloaded outside the furnace after wafers 7 are gas-cooled to a temperature under which no oxide film is formed on the surface of each of wafers 7. In the case of the conventional heat process device, however, wafers 7 are over-oxidized while boat 6 is still stayed in auxiliary air-tight tube 14.
Preliminary-opened Japanese Patent Application No. Sho 61-190929 discloses a device for preventing the wafers from being oxidized in the heat process. This device is of the fixed air shielding type but cannot sufficiently prevent the wafers from being oxidized.